In Soul Eater and Soul Eat Not!, we see a variety of different weapon types- swords, guns, scythes, etc. Based on this post, we can assume the origin of demon weapons to be from experiments. So if they are not born, but made, this means that Death and other creators originally decided what type of weapon they needed, and created accordingly. Assumedly, they could they have done this with any item, not just a weapon- thus, is there something canon that says they have to be weapons?
(Surely there were other items that could have been of use instead of pure offense. Shields, armor- with the exception of Tsubaki's and Black Star's Shield Star. This brings into question what is needed for a good team balance in a fight, which is out of scope of this question.)